To protect valuable audio data, it is often desirable to mark the data in an identifiable way. For example, if an owner of digital data desires to protect digital data against copyright infringement, the owner needs some way of showing that the data does, in fact, belong to the owner. The owner can prove ownership, inter alia, by marking the data in a known way. This is called "water marking" the data.
Marking the data in a practical way, however, is difficult, particularly because if the data is labeled using a trivial marking procedure, an infringer can easily strip off or change the mark. Furthermore, marking data by inserting or manipulating data bits corrupts the data in ways that may make the data unsuitable for its intended use.
Any analog value can be represented as a series of numbers, or digitally, and there are various formats used to represent digital data. For example, in Pulse Code Modulated (PCM) data, the arrangement of the bits is such that some bits are more important than others, ranging in importance from the most significant bit to the least significant bit. If the most significant bit is changed, the data is going to be severely corrupted, perhaps to the point where the data is unsuitable for its intended purpose. But if the least significant bit is changed, the smallest amount of corruption will result. Also, less corruption occurs if not every least significant bit is altered. For example, if every tenth least significant bit is altered, an effect is produced that is less noticeable than if every significant bit is altered. Nevertheless, altering bits in this way to water mark data can produce perceptually-intrusive results.
Another format used for digital data is called differential encoding. An example of this is Adaptive Differential Pulse Code Modulation (ADPCM). This format is selected for various types of audio data because sometimes the differential aspect of coding is desirable: one encodes not the absolute value of the sonic pressure wave, but rather the difference from one sample to the next. Because speech, for example, normally changes in small increments, the difference from one sample to the next is smaller than the absolute size of the waves. Therefore, less data is needed if only the differences are encoded. The trouble with water marking the data by encoding the difference is that values are dependent on the previously received signal; if one changes a value earlier in time, every subsequent differential value will be affected.